This invention relates to a method for purifying a polycarbonate resin solution resulting from an interfacial polymerization process. More particularly, this invention provides for the use of at least one static mixer to facilitate separation of the catalyst used to prepare the polycarbonate from the polycarbonate resin solution.
There are many well known processes for the preparation of polycarbonates from monomers. A preferred method is a two-phase interfacial process in which phosgene is contacted with aromatic bisphenols and condensed to produce high molecular weight polycarbonates.
The two-phase interfacial process results in a reaction mixture which is an emulsion of organic and aqueous alkaline phases. In order to recover the polycarbonate product from the organic phase in which it is found, the reaction mixture must first be separated into its organic and aqueous alkaline phases, and the organic phase must be further processed to remove undesirable impurities including electrolytes, catalysts, residues of chain terminators, and unreacted starting materials.
Typically, such reaction mixtures contain polycarbonate polymers with molecular weights up to 250,000 g/mol on a polycarbonate basis and brine in an amount typically around one quarter by weight of the reaction mixture. Further, the reaction mixtures may exhibit a wide range of viscosities. In conventional processes, the initial separation of the aqueous alkaline phase from the organic phase is not complete, but leaves the organic phase containing some ten percent or more of an aqueous component as well as residual impurities such as catalysts, residues of chain terminators, and unreacted bisphenols. Thus, additional steps are required to purify the organic phase.
Accordingly, it is a goal of the invention to provide a process for purifying polycarbonate resin solutions which facilitates removal of starting materials such as a catalyst from the polycarbonate resin solution.
This and other goals of the invention are obtained by separating the polycarbonate resin solution resulting from the interfacial process into a brine phase and an organic phase. The organic phase, which contains not only the desired polycarbonate resin but also unwanted residual starting material, i.e. catalyst and water, is conveyed into at least one static mixer. In a more preferred embodiment, the organic phase is combined with a wash acid water stream prior to entering the at least one static mixer. The output of the static mixer is separated into an organic phase and an aqueous phase. The aqueous phase includes a greater concentration of catalyst than the organic phase which contains the desired polycarbonate resin. In this manner the inventive process provides for extraction or removal of starting material or catalyst from the desired polycarbonate resin product.